1. Field of the Invention
This invention relates to integrated circuit fabrication and, more particularly, to a chemical vapor deposition system and method employing a mass flow controller through which cases flow from a reaction chamber to a vacuum pump, wherein the mass flow controller provides for a decrease in the time period required to evacuate the reaction chamber.
2. Description of the Related Art
Chemical vapor deposition ("CVD") is a well-known process employed during the fabrication of an integrated circuit to deposit a thin film upon a substrate. A CVD process involves introducing reactive gases, for example, silane and oxygen into a reaction chamber. Various inert carrier gases, such as hydrogen may be used to carry the reactive gases into the chamber. Reactive species within the gases adsorb onto the topological surface of a substrate which has been placed in the reaction chamber. Those adsorbed reactive species then undergo migration and reaction to form an inorganic layer across the topological surface. The gaseous by-products of the reaction are desorbed and removed from the reaction chamber, along with the unconsumed reactant gases, and the inert carrier gases.
The CVD process can take place in either pressurized or non-pressurized reaction chambers. Due to the stringent requirements of film uniformity, low pressure chemical vapor deposition ("LPCVD") reactors have gained in popularity. LPCVD reactors generally operate in the pressure range of 0.1 to 10 Torr and the temperature range of 500 to 600.degree. C. Reducing the pressure of the reactor allows the diffusivity of the reactant gas molecules to increase to an amount sufficient to eliminate mass-transfer constraints on the deposition rate. Further, low pressure operation of the reactor affords decreased gas phase reactions. As such, high quality films having relatively few impurities or contaminants can be formed upon a substrate using LPCVD.
A vacuum system is typically used to evacuate the LPCVD reaction chamber prior to introducing the reactant gases into the chamber. FIG. 1 illustrates a side view of a portion of an exemplary LPCVD system, deemed Alpha 858S which is commercially available from Tokyo Electronics Limited Co. The LPCVD system includes a reaction chamber 10, a primary outlet conduit 12, and a secondary outlet conduit 16 which are all in gaseous communication with each other. Primary outlet conduit 12 connects reaction chamber 10 to a vacuum pump. A primary valve 14 is positioned within primary outlet conduit 12 for controlling the flow of gases through primary outlet conduit 12. A secondary outlet conduit 16 is connected to a first point 32 of primary outlet conduit 12 located upstream of primary valve 14. The diameter of secondary outlet conduit 16 is substantially smaller than that of primary outlet conduit 12. As secondary outlet conduit 16 passes away from that first point of primary outlet conduit 12, it diverges into lines 18 and 20. Line 18 extends to a second point 34 of primary outlet conduit 12 located downstream of primary valve 14 while line 20 exhausts gas from the LPCVD system. A secondary isolation valve 22 and a manually adjustable needle valve 24 are disposed within line 18. The LPCVD system also includes a cold trap 26 for condensing species within the gases passing therethrough that could be corrosive and hazardous to, e.g., parts of the vacuum pump system.
Evacuation of reaction chamber 10 involves a slow pump step and a fast pump step. The slow pump step is used to reduce the pressure within reaction chamber 10 from atmospheric pressure (760 Torr) to about 1 Torr (i.e., the "crossover" point). In the slow pump step, primary isolation valve 14 is closed and secondary isolation valve 22 is opened. Thus, gases are drawn from reaction chamber 10 to the vacuum pump through secondary conduit 16 and line 18 without passing through primary valve 14. Once the crossover point has been reached, primary valve 14 is opened to permit the gases to pass through the main section of primary conduit 12 to the vacuum pump. In this manner, the fast pump step is performed to reduce the pressure within reaction chamber 10 from 1 Torr to about 1 to 10 milliTorr. The slow pump step allows a regime of vacuum at the crossover point to be achieved within reaction chamber 10 without causing high turbulence in the flow of the gases. Opening primary valve 14 when the pressure within reaction chamber 10 is about 1 Torr is less likely to cause the outer shell (e.g., a quartz tube) of reaction chamber 10 to fracture. Otherwise, if primary valve 14 is opened when reaction chamber 10 is at atmospheric pressure, the outer shell of reaction chamber 10 might implode from the large amount of suction generated. The relatively large opening (e.g., 4 inches) inside the valve in combination with the great pressure differential would cause the gases to flow from reaction chamber 10 at a relatively high rate.
Needle valve 24 is initially manually adjusted to partially restrict the flow of the gases passing through line 18 during the slow pump step. Since needle valve 24 is manually adjusted, its initial setting unfortunately may be inconsistent from one LPCVD maintenance procedure to the next. The setting of needle valve 24 is chosen to allow reaction chamber 10 to be pumped down to the crossover point as quickly as possible without introducing an excessive amount of particles into the gases exiting reaction chamber 10. Such particles may comprise the products (e.g., silicon nitride, silicon dioxide, and polycrystalline silicon) and the by-products of the LPCVD reactions. The inner wall of reaction chamber 10 may become coated with the products and the by-products over time. Due to the initial impact on the shell of reaction chamber 10 when secondary isolation valve 22 is opened, some of the products and the by-products accumulated thereon may fall into the gas stream as particles. Those particles can contaminate and damage semiconductor topographies during subsequent processing steps, rendering ensuing integrated circuits inoperable.
As the slow pump step progresses, the pressure differential between reaction chamber 10 and the vacuum pump decreases and the flow rate through needle valve 24 decreases. Unfortunately, the initial setting of needle valve 24 can only be changed manually, and thus remains fixed unless a person continuously monitors and adjusts it. The cost of reserving someone for only the operation of needle valve 24 is typically not feasible to the integrated circuit manufacturer. Absent continuous adjustment of needle valve 24, the flow rate of the gases passing therethrough will drop rapidly, increasing the time period required to reach the crossover point. Consequently, the amount of time required to deposit a film onto a substrate using LPCVD is increased. Therefore, using needle valve 24 to control the flow of the gases through line 18 undesirably limits the level of throughput that can be achieved by the integrated circuit manufacturer. The throughput may be increased at the risk of introducing more particle defects to the integrated circuits being fabricated. That is, the initial setting of needle valve 24 may be adjusted to provide for a greater initial flow rate of the gases through the needle valve. However, the additional pressure shock to reaction chamber 10 may result in a significant increase in the amount of particles entering the gases within the chamber.
It would therefore be of benefit to develop a technique for strictly controlling the flow rate of gases through the secondary outlet conduit of the LPCVD system. In particular, a constant flow rate must be maintained within the secondary outlet conduit so that the crossover point may be reached more quickly. In other words, the amount of time required for the slow pump step must be reduced to increase the throughput of the manufacturer. Accordingly, it would be desirable to replace the manually adjustable needle valve with a valve that could automatically control the flow rate without requiring constant readjustment by a person. In addition, the initial flow rate of the gases passing through the secondary outlet conduit must be minimized to lower the number of particles entering into the gas stream.